Low-temperature synthesis of polycrystalline germanium (poly-Ge) thin films is of great interest in thin-film photovoltaic and electronics applications. Low-temperature synthesis of polycrystalline semiconductor thin films (Si or Ge) on amorphous substrates has important applications in thin-film transistors and solar cells. Recently, (poly-Ge) thin films have been investigated as a low-cost substitute for single crystalline Ge substrates. Ge has good lattice matching with III-V semiconductors, and poly-Ge substrates have been shown to seed epitaxial growth of GaAs or Ge overlayers for polycrystalline single-junction or multijunction solar cell fabrication. Moreover, as-grown poly-Ge films can serve as building blocks for thin-film transistors, a key device in advanced large-area electronics, such as flexible displays.
Metal-induced (e.g., Al, Ni) crystallization (MIC) of amorphous semiconductor films can form polycrystalline films at temperatures much lower than those required in the absence of a metal catalyst. Amorphous Ge (a-Ge) thin films crystallize at 150-250° C. when Ge is in direct contact with Al. This opens up the possibility of crystalline semiconductor deposition on flexible polymeric substrates, whereas a-Ge solid-phase crystallization requires temperatures significantly higher than 300° C. However, a-Ge/Al interfaces exhibit uncontrolled crystallization with growth of randomly oriented Ge nanocrystallites mixed into the Al layer. For many applications, preparation of a polycrystalline semiconductor layer on the surface of a large-area substrate with controlled crystallite size and crystallographic orientation is crucial, and this is difficult to achieve with existing methods.
What is needed is a method of forming oriented crystalline Ge structures on thin films using low-temperature processing for use in thin-film photovoltaics, thin-film transistors and large-area electronics.